But KFC's target diners insisted the chickens were not being killed in the right way and said they would stay away.

And furious non-Muslim customers have set up Facebook groups protesting that the trial branches, which also ban pork, have dropped their favorite bacon-topped Big Daddy burger from menus. Groups with names such as "Against the KFC Halal Trial" and "No Halal at Colne KFC" -- referring to a branch in the Lancashire, northern England, town -- were rapidly attracting members.

For meat to be halal, the animal must be alive when its throat is cut as a verse from the Koran is recited.

KFC insisted their methods met the approval of the Halal Food Society. But Islamic leaders disagreed, saying the pre-stunning of animals in the chain's mechanical process means a third were already dead at the point of slaughter. And the fact the prayer is played over a speaker means each bird it not blessed individually as it is killed.

They have now threatened to warn the U.K.'s 2.4 million Muslims not to eat KFC meat, and will meet with the fast food giant Wednesday to question how it is killing chickens sold as halal.